This invention relates to small synchronous motors and more particularly to small synchronous motors which can start in a short period of time at the frequency of a commercial power source.
The term "small motors" as herein used, is intended to designate motors of the character described below. One typical example of such a motor is one having a housing diameter of the order of 50 mm. and a housing length of the order of 60 mm.
Electric motors which are small in size and run at a constant rotational speed have been in demand and used for devices such as tape recorders and record players. In order to satisfy this demand an alternating current electric motor, for instance, an electric motor of shading coil type which can rotate at a constant synchronous speed has been employed for tape recorders, record players and the like.
However, the electric motor of this type has a small starting torque. Therefore, even in the case where a light load such as a turntable is applied to the motor, the motor takes several seconds before it reaches a predetermined or required speed. Thus, the starting operation of the motor cannot be accomplished within a short period of time. Furthermore, in order to increase the starting torque of the motor, it is necessary to make the size of the motor greater.
While, on the other hand, the employment of a DC machine would provide a greater starting torque, an intricate servo-control-system must be provided in order to hold constant the speed of the DC machine after the starting operation thereof.